In the technical field of laser leveling, a laser rotator or a line laser is used to span a leveling plane which can be horizontal, vertical or tilted by a desired angle. The laser plane is often detected by a laser receiver, which detects the striking position of the laser inside a detection window to determine the receiver's position relative to the laser plane. The detection window, or receiver window, is the area at the laser receiver device, which is embodied to detect the light beam, in particular the position of the light beam's striking point within the detection window for determining the relative position of the laser-beam's axis and the receiver device in at least one direction. In many embodiments, the center in the half of the length is the desired level of the laser beam and the receiver indicates a deviation from this ideal position by indicative or quantifying means. The electronic reception of the laser beam allows an extension of the range of the laser leveling, usability under sunlight conditions, etc. Many types of laser receivers are known, with many different functional, performance and cost advantages compared to their competitors. The devices can comprise many additional features like optical level indication, acoustic level indication, distance determination, angle determination etc., which are not the primary scope of the present invention.
Some examples of such laser leveling implementations are U.S. Pat. No. 6,435,283 showing a rotating single beam laser transmitter, or U.S. Pat. No. 4,756,617 showing a continuous 360° laser plane generated by directing a laser at a conical surface, wherein typically an amplitude modulated laser source is used. Those two principles of projecting a laser line are also referred to as line lasers with a scanning line or line lasers with a continuous laser line, wherein the light can be modulated.
A rotating or scanning laser beam transmitter produces a beam that sweeps past the receiver and generates laser pulses of short duration but of high bandwidth in the detection window. In contrast, continuous laser transmitters generate a continuous laser signal of a narrow bandwidth at the modulation frequency. Although the sensor element of the receiver (and many other parts of the receiver) can be the same for both types of laser levels, the signal conditioning and evaluation in the receiver is different. The invention presented herein relates to rotating beam transmitters, but is also valid for modulated continuous beam laser transmitters.
In many cases of use, the laser receiver is attached to a leveling rod or to machinery which has to be leveled. The point of intersection of the laser plane at the reception window of the receiver is determined by measuring the striking position of the beam's optical axis within the receiving window. This position is indicated to the user, in particular as a deviance from a desired striking position at the desired leveling position.
There are multiple techniques known to determine the striking position of the laser in the receiver window. One approach of determining the position of a light beam is by usage of photodiodes or arrays of photodiodes.
U.S. Pat. No. 3,649,122 describes a line of separate adjacent photo electric units connected in parallel to each other and connected in series with identical resistors. The photocurrent measured between the thus resulting poles gives the position of the point at which the laser beam strikes the receiver relative to the length of the line of photo electric units. Light guides are used to increase the filling/density of the detection window with photo electric units. For the arrangement of the photo electric units, this receiver only yields a stepwise linear response.
U.S. Pat. No. 3,894,230 describes an aligned array of parallel photo detecting devices where each photo-detecting device corresponds to an increment in position. The receiver indicates which photo detection unit first receives a light signal above a predetermined level. A Plexiglas rod is aligned with the sensor array to form a cylindrical lens, increasing the amount of light falling onto the detection array. The apparatus further employs the use of optical filters to reduce the influence of ultra-violet light, bright sunlight or other sources of light. This receiver assumes that the laser beam is circular and that photo detecting units are sufficiently spaced apart. Therefore the range of detection of this receiver is limited.
EP 1,484,577 describes a receiver for amplitude modulated laser beams (100 kHz to 10 MHz) by an intermittent array of (parallel) photodiodes that are connected to a phase shifting circuitry. A light diffusing sensor window is arranged in front of the intermittent array of photodiodes. The light diffuser spreads the laser light in such a manner that when the laser beam impinges on the center between two photodiodes, both diodes are equally illuminated. By this structure this receiver can use fewer photodiodes; however this receiver is only partly linear.
U.S. Pat. No. 7,372,011 describes a linear array of (parallel) photodiodes with associated weighting circuits. The weighting circuits are used to determine whether the receiver has been struck by a laser beam or by a strobe light. To drain the low frequency current generated by sunlight, the receiver uses inductors referenced to ground.
U.S. Pat. No. 7,339,154 and JP 04046282 show a sensor stage with a parallel array of photodiodes weighted with a circuit based on an arrangement of resistors in series. The load resistors Rh and Rl are preferably dimensioned such that the sum of the weighing resistors is considerably larger than the load resistors.
Besides single photodiodes as used above, also BiCells are used, as dual photodiodes, placed on a rectangle with a typically long aspect ratio and which are electrically as well as optically separated by a diagonal, forming two congruent triangles (Bi-Cell). When the laser beam strikes at the upper/lower side of the rectangle, the first photodiode covers the laser beam with a small/large area, whereas the second photodiode is covered with the remaining large/small area. The relationship between the difference of both signals and the sum of both signals is linear. For example, U.S. Pat. No. 4,676,634 shows a BiCell and alternative two photocells in butt to butt arrangement with a beam averaging and deadband selection for on-grade stabilization.
U.S. Pat. No. 4,756,617 describes the usage of so called BiCells for a linear determination of the position where the laser beam impinges on the receiver. The use of inductors is described for bypassing the DC and low frequency signals to ground (sunlight protection). Further, the use of amplitude modulated laser beams is described to increase the peak strength of the laser beam without increasing the regulated RMS (root mean square) value which allows an increase of the maximum distance range of the receiver.
U.S. Pat. No. 4,830,489 describes a BiCell based laser receiver for linearly determining the relative height of the laser beam. The azimuth position of the receiver is determined at the rotator by means of a back reflected signal by the receiver. A radio is used to communicate the azimuth position as well as the distance of the rotator to the receiver. For distance measurement, time of flight as well as phase shift measurement methods are proposed.
U.S. Pat. Nos. 4,907,874 & 4,976,538 both describe a modified BiCell sensor through a parallel arrangement of the individual interdigitated photo elements combined in two resulting elements with two signals.
U.S. Pat. No. 6,133,991 describes multiple BiCell sensors stacked on top of each other with different sensor shapes and sensor arrangements to reduce the shading effect. For effecting a linear sensor array concatenating the same sensor element, the contributing sensor elements are electronically used in parallel and requiring for each a separate signal processing path. In an alternative configuration, different elements are required at each position and multiple sensors are electrically interconnected in parallel.
U.S. Pat. No. 6,747,266 describes an arrangement using an optical filter with a lenticular part capable of expanding the laser beam in scanning direction and a diffusing part capable of diffusing an expanded laser beam with the advantage that a wide laser beam as well as a narrowly focused (scanning) laser beam can be detected by a divided Bi-cell sensor.
Also, the usage of lightguides in leveling laser receivers is known. For example in DE 195 40 590 an N-type or V-type line shaped optical detection unit is described. The detection units can consist of arrays of photo electrical sensors, bundles of optically transmitting glass or plastic fibers which are aligned in a line at the detection window or a specially made bar consisting of optical material able to conduct the orthogonally impinging laser light to a photo electric sensor at the end of the bar. By using the bars in N-shape or V-shape and measuring the time difference between receipt of signals on either bar, the relative height of the laser beam on the optical receiving elements is determined.
U.S. Pat. No. 7,394,527 and US 2003/0174305 both describe a rotator with at least two signal beams in a given relationship to each other for determining the distance from the receiver to the rotator. The receiver is designed for time resolved reception of these signal beams. Furthermore a receiver is described with two photo sensors on both ends of a light conductor which allows measuring the point where the light beam impinges the light conductor. Using two receivers arranged in a fixed distance allows determining the distance to a rotator by only a single beam.
Differentiating two signal generators by emitting at the receivers can take place via different rotational frequencies, coding of the emitted signals, selection of a suitable emission spectrum and similar.
Another known element for the determination of a light point is a PSD (short for Position Sensitive Device), which is well known in industry and is for example used in triangulation sensors as device to detect the position of a (reflected) laser beam.
PSDs according to the understanding of the present application are devices like those based on the lateral photoelectric effect, also referred to as isotropic PSDs or Lateral Effect Photodiodes (LEP) or MOS-Type PSDs. As the presented readout circuitries imply, in particular non-segmented PSD substrates are used, which have two opposed, balanced outputs. The signals on the outputs are dependent on the position of a light beam on the PSD's active surface. The two output signal are reversely dependent on the position (such output behavior is sometimes also referred to as symmetrical output). Fast-readout CCD or CMOS arrays, which are sometimes also referred as PSDs—since they can also be used to determine the position of a light spot projected on them—are not meant, in particular since their electrical connections and readout characteristics are different.
For ease of understanding, the PSDs in the explanations of the present text are one-dimensional PSDs, although a skilled person is aware of the fact that the same principles are also applicable on a second direction of two dimensional PSDs, wherefore also the evaluation of two dimensional PSDs is part of the present invention.
Although the usage of PSDs for position detection has advantages, the main problem is their low saturation threshold, wherefore, in outdoor conditions with bright sunlight, PSDs have not been used thus far. In particular in outdoor laser leveling applications, PSD sensors suffer from the fact that they easily saturate under sunlight and—relative to photodiodes—have lower signal strength at far distances between the receiver and the laser source. For those reasons, PSD sensors have not been used in a commercial implementation of a laser receiver for construction sites.
The signal response of the PSD is somehow similar to that of a BiCell arrangement: providing two continuous signals that increase/decrease with changed position of the incident laser beam. However, there are several technical differences in the use of a BiCell compared to a PSD, for example:    1. There is a phase delay between the signals created by each photodiode of a BiCell. This phase delay has disadvantages during the digitalization of a pulse and is cause to some non linearity.    2. The triangular shape of each diode in a BiCell causes some non linearity when being struck by a round or elliptical laser beam because of the difference in point of gravity of the triangular sensor areas and the laser beam intensity distribution.    3. BiCells typically require a larger detection area than PSDs, which results in higher sensor cost and more sunlight saturation.    4. When the BiCell is not aligned orthogonally to the laser beam, and a part of the sensor is covered by the receiver housing, the sensor will give a wrong height reading. This is also referred to as shading effect. Many solutions have been proposed to solve this inherent BiCell sensor problem.
In leveling applications using “intelligent leveling rods”, it is desired to cover large height differences, for example 10 cm or more.
Standard, off the shelf PSD devices with long detection areas are not available, for example PSDs having an active detection length greater than 5 cm or 7 cm or even more than 10 cm. Those few devices available suffer from saturation problems by background light etc. and can not achieve the desired performance which is required for a leveling receiver.